Electrochemical cell with stepped voltage output

ABSTRACT

An electrochemical cell is provided having a stepped voltage output during its lifetime due to multiple composition anode and cathode construction. The stepped output voltage provides a beginning of life (BOL) voltage indication, a voltage output indicative of the main capacity of the cell, and an end of life (EOL) output voltage indication warning the user that the total battery capacity is nearly exhausted.

United States Patent Fester et a1.

Output voltage ELECTROCHEMICAL CELL WITH STEPPED VOLTAGE OUTPUTInventors: Keith E. Fester, Roseville; Richard L. Doty, White Bear Lake,both of Minn.

Assignee: Medtronic, Inc., Minneapolis,

Minn.

Filed: Nov. 15, 1971 Appl. No.: 198,823

U.S. Cl 128/419 P, 136/111 Int. Cl. A61n l/36 Field of Search 136/111,107, 20,

References Cited UNITED STATES PATENTS 2,829,189 4/1958 Coleman et a1136/107 2,942,052 6/1960 Bourke et a1.

3,057,356 10/1962 Greatbatch 128/422 2,772,321 11/1956 Ensign 136/120 R3,104,990 9/1963 Solomon et a1 136/20 Primary ExaminerAnthony SkaparsAtzorneyEverett J. Schroeder et a1.

[57] ABSTRACT An electrochemical cell is provided having a steppedvoltage output during its lifetime due to multiple composition anode andcathode construction. The stepped output voltage provides a beginning oflife (BOL) volt age indication, a voltage output indicative of the maincapacity of the cell, and an end of life (EOL) output voltage indicationwarning the user that the total battery capacity is nearly exhausted.

4 Claims, 3 Drawing Figures 2,708,683 5/1955 Eisen ..136/X 11/1956Fleischer ..136/24 010 20 3o 6O so Depletion of total power capacityPATENIEB s5? 1 'I ma I 0 20 3O 4O 5O 7O 8O Deplefidn of total powercapacity INVENTORS. KEITH FES Q/CHAIQD L. D

BY MJZMF W ATTORNEYS.

n ELECTROCHEMICAL CELL WITI-I STEPPED VOLTAGE ()UTPUT The presentinvention is directed both to primary and to secondary electrochemicalcells and is more specifically directed to such cells wherein thevoltage output of the cell changes in distinct voltage steps as the cellis depleted. That is, when the cell is in a fully charged condition thevoltage across the terminals will be within a first value, the beginningof life (BOL) voltage, during the initial period of discharge and willundergo a stepped drop in voltage to a main operating voltageencompassing the major portion of the total cell power output. After theexhaustion of the ingredients forming the power source for the majorpart of power output a second stepped voltage drop will take place to athird voltage output, the end of life (EOL) voltage, which provides anindication to the user of the cell that the total power capacity of thecell is nearly gone.

It is also an object of the invention to provide electrochemical cellswherein a stepped output voltage is provided for an indication of thebeginning-of-life (BOL) or end-of-life (EOL) cycle alone without havingboth cycles present.

IN THE DRAWINGS FIG. l is a cross-sectional view of a button cell inaccordance with the invention;

FIG. 2 is a plot of output voltage vs power capacity of cells inaccordance with one form of the invention; and

FIG. 3 is a circuit of a pacer including an electrolytic cell inaccordance with the invention.

While the principles of the invention are equally applicable to bothprimary and secondary cells the invention will be described withparticularity in its use in the primary cells. The most common primarycells are. the common type used in flashlights of the so-called dry celltype and the button cells typically found in instruments such as hearingaids and photographic equipment. As the purchasers of both of thesetypes of cells are aware cells that are purchased through retail outletsfrequently have had their total power output depleted to a large extentthrough internal dissipation as a result of prolonged storage beforeactual use. The purchaser rarely has the facilities necessary for makinga determination as to the state of charge of the cell he is purchasing.All that he can tell is that the cell is supposedly a new cell. Someretail stores provide a battery checker which will give an appropriateindication as to whether the battery has long passed its full chargecondition, but these types of checkers are not entirely satisfactory inthat they do not indicate with any precision the total remaining lifewithin the battery.

For most uses it is not critical that the battery have a known totallife. The user of the battery generally stands to lose only in the sensethat he has paid full price for a half-empty cell. For other users thisis not the case.

It is contemplated that the present invention will find its mostimportant immediate use in the field of power supplies for medicalcardiac pacers. It is known in the art that such pacers are commonlyprovided with a power source and electronics that in some instances areactually embedded within the body of the patient. The installation ofsuch a pacer within the body of the patient is relatively costly.Therefore, it is of utmost importance that the battery used to power thedevice be at the top of its charged condition at the time it isinstalled so as to insure that the greatest possible lifetime can beexpected from a single implantation. In accomplishing this purposeelaborate procedures have been developed for insuring that the powersupplies are in fact at their peak charge condition. One such techniquehas been to x-ray the electrochemical cells to insure that the size ofthe anode and cathode is within the tolerance required for a fullcharge. While such a procedure works satisfactorily with certain typesof electrochemical cell configurations it is not a technique which isapplicable to all types of cells.

Even when the cells are known to be at a peak power state when they areinstalled within the patient there are many variables which tend toshift the life expectancy of the battery in a somewhat unpredictablemanner. Therefore, a large margin for error must be provided for in aschedule of replacement of the implanted unit in order to insure thatthe patient does not encounter difficulties arising from a failure ofthe power source. What this means in practical terms is that the unitmust be removed from the patient at points in time well ahead of thereasonable life expectancy of the batteries. This, of course, greatlyincreases the annual ex-' pense which the patient must pay.

In accordance with our invention we construct electrochemical cells in amanner so that at the full charge condition the output of the individualcells (or correspondingly of an array of cells in batteries) is at avoltage above the normal operating voltage of the pacer. Thus merely bychecking the voltage across the electrodes of the cell one can determinewhether the cell is in a fully charged condition. This initial voltageand ampere hour output of the cell is for only a relatively smallpercentage of its total ampere hour outputusually in the range of l to15 percent of the total ampere hour output. Following this initialoutput the voltage of the cells decreases stepwise to its principaloperating voltage which comprises in the vicinity of about percent ofthe total power capacity of the cell. When the cell is near the point oftotal depletion the voltage output drops to a third value which gives anindication that the total power capability of the cell is nearingexhaustion. The third stage voltage and ampere hour output will be from1 to 15 percent of the total ampere hour of the cell.

it will be apparent that initial voltage across the terminals of thecell can be readily checked before implantation. In the patient itself"the increased voltage can, by appropriate circuit design of the pacer,act to increase the beats per minute of the heart to some value abovethe nominal desired, although well within a normal excursion ofheartbeat. After this initial depletion has taken place the voltage ofthe cell to the pacer causes the heart to drop to a predetermined pointof beats per minute (typically 72) and continues the heartbeat at thisrate throughout the major portion of the cell life. When the mainportion of the cell is exhausted the voltage drops to the third level.The patient will be able to readily ascertain that his heartbeat hasdeclined to some predetermined value less than 72 and yet well withinthe range necessary to sustain life and reasonably normal activity. Hewill be alerted at this point that his pacer power supply is nearingdepletion, and yet, will have adequate time to provide for replacementof the unit. Even if the patient is not aware of the shift in hisheartbeat rate periodic examination will readily reveal this fact to thedoctor. Of course, when the pacer is external to the patient periodicvoltage checks will supply the information of the cells state of chargedirectly.

We accomplish the purposes of the invention outlined above through theuse of an electrochemical cell which has an anode, a cathode and asuitable electrolyte all within a casing wherein either the anode or thecathode or each of them consists of a plurality of different materialseach of which has an E., that differs from the other materials whichcomprise the anodes and cathodes. While we anticipate that our inventionwill find use in a variety of different types of electrochemical cellsit will be described herein with principal reference to thealkaline-type of sealed button cell presently used in the pacerindustry. While the invention will be described with great particularityto the alkaline cells it will be apparent to those skilled in the art ofelectrochemical cells that the principles of the invention are equallyapplicable to other types of cells. Likewise, while the invention willbe described with particularity in regard to cells that are sealedagainst gas and vapor loss from the cell the principles will be likewiseapplicable to cells which are vented to the atmosphere.

In the field of alkaline-type cells the most common cell available isthe zinc/mercuric oxide cell wherein the zinc metal is present in theform of an amalgam and the mercuric oxide is in powder form and isinterspersed with silver particles which tend to absorb the mercurywhich is produced at the cathode and increase the conductivity of thecathode active material. For the sake of efficiency such cells aredesigned so that the quantity of zinc metal present is electrochemicallynearly the same as the electrochemical equivalent of the cathodicdepolarizer material. When an excess of either zinc or mercuric oxide ispresent the cell is considered to be either cathode limited or anodelimited.

various additives depending upon the choice of anode and cathodematerials.

A typical cell in accordance with the invention is shown in a somewhatschematic cross-sectional view in FIG. 1. It shouldv be appreciated thata wide variety of cell configurations is possible using the principlesof the invention. FIG. 1 is merely to illustrate one common form of cellthat has been adapted to include the present invention. Turning to FIG.1 it will be seen that the cell consists of a lower cup member 1 1 andan upper cup member 12 which are nested together as shown. Cup members11 and 12 are desirably of a metal that will be essentially inert to theingredients contained within the cell itself, such as nickel metal orsteel, that has been plated with a layer of nickel. Members 11 and 12may also be copper. The open edges of cups l1 and 12 have been formedinto a crimp seal 18 with an intervening spacer vl3 of a resilientnonconductive material. Suitable materials are nylon, polyethylene,rubbers and the like. The material chosen should be one which issubstantially inert to the electrolyte that is contained within thecell. For an alkaline cell this material should, of course, be capableof forming an essential hermetic seal and be inert under long-termexposure to strong caustic.

Pressed into the base portion of shell 12 is an anodeactive material 14which will be described in greater detail in the examples below. Pressedinto shell 11 is a cathode-active material 15 which will likewise bedescribed with greater particularity hereinbelow. Intermediate materials14 and 15 is a barrier member 16 which operates to prevent transfer ofmaterial from anode 14 to the cathode material 15. Material 16 can beany of the conventional materials used for this purpose, such asmicroporous polyvinyl chloride, cellophane or grafted polyethylene. Itshould, of course, be a material which is substantially unaffected byexposure to the electrolyte within the cells. Also contained within thecell 10 is a quantity of an electrolyte substance generally designated17. An electrolyte absorbent material commonly used in cells of thistype can be included to hold the electrolyte.

As can be seen from the above description the present invention in itsbroadest aspects is substantially the same as the prior artconstruction. The invention differs from the prior art in its choice ofmaterials for the anode-active material 14 and the cathode-activematerial 15. These will now be described in greater detail.

EXAMPLE 1 An anode-active material is formed by blending 2.0 grams ofzinc powder with 0.2 grams of cadmium metal powder. A quantity ofmetallic mercury (approximately 0.2 grams) is added to this mixture toincrease the conductivity and to decrease the tendency for the zinc tocorrode in the cell. The powders are intimately blended together andpressed to form a pellet within the lower confines of cup 12. Acathode-active material is made up using 7.024 grams of mercuric oxideand 0.010 grams of silver oxide. This material is likewise formed into apellet in the base of cup 11. An electrolyte of approximately 30 percentby weight potassium hydroxide is then introduced into the cell so as tocontact both the anodeand cathode-active materials and the cell issealed into a configuration like that of FIG. 1. The initial outputvoltage of the cell formed from these materials is that of thezinc/silver oxide couple and is approximately 1.55 volts. At a currentdrain of 20 microamps the voltage output of the cell would re main atthis initial voltage for approximately 9 days. Following exhaustion ofthe zinc/silver oxide couple the voltage declines to approximately 1.35volts-the output of the zinc/mercuric oxide couple. With the quantitiesof materials utilized in forming the cell and with a drain of 20microamp current on the cell the voltage would remain at substantiallythis level for about 9.3 years assuming electrochemical efficiency ofpercent. Of course, under higher current drain the time element would beconsiderably reduced. After exhaustion of the zinc/mercuric oxide couplethe voltage output declines to approximately 0.90 volts which isrepresentative of the cadmium/mercuric oxide couple. With the quantityof materials utilized in forming the cell and assuming again a currentdrain of 20 microamps the voltage would remain at this level forapproximately 6% months.

Of course, if a plurality of such cells were connected in series theinitial voltage would be the sum of the number of cells inse'ries andeach incremental step would be the voltage output of the sum of thestate of the cells in the series. For example, if three cells wereseries joined then assuming the cells each depleted one couple atdifierent intervals the idealized stages of depletion would be: 4.65volts, 4.45 volts, 4.25 volts, 4.05 volts, 3.60 volts, 3.15 volts, and2.70 volts as the cells each reached a final voltage stage in theiroutputs.

EXAMPLE II A number of cells may be prepared using the constructiondescribed in reference to FIG. 1 except that the cathode-active materialis formed of a mixture of silver oxide and silver peroxide powders. Thesilver .peroxide is present in quantities ranging from I to 15 percentof the silver oxide. The electrolyte is, as in Example I, a 30 percentby weight solution of KOH in water. The anode is the same as in ExampleI. As an alternative to blending the oxide and peroxide of silver onecan produce the silver peroxide by electrolytic means by overcharging asilver oxide cathode material. The combined quantity of Ag O and Ag O isdesirably slightly in excess of the electrochemical equivalent of thezinc and cadmium to produce a cell that is anode limited. The 801.voltage of the cell is that of the zinc- /silver peroxide couple atapproximately 1.85 volts at low current drain conditions. Afterexhaustion of this couple the voltage drops to the zinc/silver oxidecouple with an output of 1.55 volts for the principal portion of thecell life. EOL indication is provided by the cadmium/silver oxide coupleat a voltage of 1.15 for the end portion of the cell life.

EXAMPLE III Another example of the invention utilizes cells preparedusing zinc as the anode material and a mixture of silver peroxide,silver oxide and mercuric oxide as the cathode-active material. Silverperoxide constitutes 2 percent of the total electrochemical equivalentof the cathode-active material and the quantity of mercuric oxideconstitutes percent of the electrochemical equivalent cathode-activematerial. As in the preceding examples the total cathode-active materialis electrochemically slightly in excess of the quantity of zinc. The BOLvoltage is 1.85 volts (zinc/silver peroxide couple) which decreases to1.55 volts (zinc/silver oxide couple) after depletion of the silverperoxide. The EOL voltage produced after exhaustion of the silver oxideis that of the zinc/mercuric oxide couple or 1.35 volts.

EXAMPLE IV A cell is prepared as in the preceding examples with theexception of the choices of materials used for the anode and cathodes,The anode is prepared from powdered indium and cadmium metals while thecathodeactive materials are a mixture of silver oxide and mercuricoxide. The couples are then:

BOL In/Ag,0 1.35 volts Principal In/HgO 1.18 .volts EOL Cd/HgO 0.90volts As in the preceding examples the material determining the BOLcouple life (Ag O) is desirably present in a quantity equivalent to fromabout 1 percent up to about percent (preferably l-5%) of total cellcapacity and the EOL couple life determining substance (Cd) desirably isabout 5 percentof the total cell capacity.

EXAMPLE V' A further example of our invention is a cell where the anodeis zinc and indium and the cathode is a mixture of Ag,() and HgO. Thecorresponding couples will be:

BOL Zn/Ag,0 1.55 volts Principal Zn/HgO 1.35 volts EOL ln/HgO 1.18 voltsAs in the preceding examples the quantities of materials used to formthe anodes and cathodes is dictated by the proportion of total cell lifein each couple.

EXAMPLE VI Still further examples of our invention are cells where theanode is zinc blended with either cadmium or indium and the cathodematerial is mercuric oxide and manganese dioxide. Where cadmium is theblended material with zinc the couples will be as follows:

BOL Zn/MnO, 1.45 volts Principal Zn/HgO 1.35 volts EOL Cd/HgO 0.90 voltsWhen indium is the blended material the couples would be:

BOL Zn/MnO, 1.45 volts Principal Zn/HgO 1.35 volts EOL 1.18 volts Theprinciples of our invention should now be clear to those skilled in theart of electrochemical cells. Likewise, it will be readily apparent that.a wide variety of materials are suitable for use in alkaline cells inaddition to those set forth above. For example, other cathode-activematerials of the oxide type such as copper oxide and manganese dioxidecan be used while nonoxide cathode-active materials such as mercuricsulfate can be used. Choice of the active materials to be used will bedictated by matters such as the voltage output desired, tendency of theactive materials or their reaction products to cause internaldissipation of the cells energy and the like. Various known means can beused to increase conductivity of the electrodes such as mercury metal,silver metal, graphite and metallic grids.

The well-known nickel-iron alkaline cell is a further example of a cellwhich can be modified using the principles of the invention by includingin the anode cadmium to form an EOL indicator.

The electrolyte for alkaline cells may be an aqueous solution of sodiumhydroxide as well as potassium hydroxide and the concentration can bevaried over broad limits although typically will be in the range of30-50 percent by weight.

In accordance with the purposes of the invention it is important thatthe electrolytic couplescomprising the cell be chosen so that thevoltage steps be sufficiently large to be readily detectable and smallenough so that the voltage output of the cell remains at useful levels.As shown in the examples the voltage charge from one couple to the nextshould be at least 0.1 volts and should be less than 0.5 volts.

The anodes which use two different metals can be formed by plating thedifferent metals over one another rather than using powdered metals asdescribed in the examples.

It should be recognized that some of the cells of the examples abovewill be preferred for use in pacers over others. For example, silverperoxide is less stable than silver oxide so cells made utilizing theperoxide would be less desirable for the purpose of pacer power suppliesbecause of the greater instability. However, for other uses of lesscritical nature silver peroxide containing cells are satisfactory.

Referring to FIG. 3 there is shown a circuit diagram 20 of a pacer inaccordance with the Greatbatch U. S. Pat. No. 3,057,356. An electrolyticcell power source 22 has been provided with test leads 23 which can beused to directly measure the output voltage and thus the state of chargeof source 22. As the specific operation of the circuit does not form apart of our invention the reader is directed to the Greatbatch patentfor further description if desired.

The embodiments of the invention in which an exclusive property orprivilege is claimed are defined as follows:

1. In combination an electrical circuit defining a medical cardiac pacerand an electrolytic cell as a power source therefor, said cellcomprising a casing, an anode and a cathode and an alkaline electrolytepositioned within said casing, lead means connecting said cell anode andcathode to said pacer electrical circuit, said anode and said cathodehaving a plurality of electrochemically active materials defining withsaid electrolyte first, second and third electrolytic couples, saidfirst couple having a higher E than said second couple and said secondcouple having a higher E, than said third couple, the first and third ofsaid couples each comprising from about 1 to about 15 percent of thetotal ampere hour capacity of the cell.

2. A combination in accordance with claim 1 wherein said electricalcircuit includes test terminals for determining the voltage output andthus the state of charge of said cell.

3. The combination in accordance with claim 1 wherein theelectrochemically active material of said anode is zinc and theelectrochemically active material of said cathode is a mixture of silverperoxide, silver oxide and mercuric oxide.

4. The combination in accordance with claim 1 wherein theelectrochemically active materials of said anode are zinc and cadmiumand the electrochemically active material of said anode is a mixture ofsilver per oxide and silver oxide.

1. In combination an electrical circuit defining a medical cardiac pacerand an electrolytic cell as a power source therefor, said cellcomprising a casing, an anode and a cathode and an alkaline electrolytepositioned within said casing, lead means connecting said cell anode andcathode to said pacer electrical circuit, said anode and said cathodehaving a plurality of electrochemically active materials defining withsaid electrolyte first, second and third electrolytic couples, saidfirst couple having a higher Eo than said second couple and said secondcouple having a higher Eo than said third couple, the first and third ofsaid couples each comprising from about 1 to about 15 percent of thetotal ampere hour capacity of the cell.
 2. A combination in accordancewith claim 1 wherein said electrical circuit includes test terminals fordetermining the voltage output and thus the state of charge of saidcell.
 3. The combination in accordance with claim 1 wherein theelectrochemically active material of said anode is zinc and theelectroChemically active material of said cathode is a mixture of silverperoxide, silver oxide and mercuric oxide.
 4. The combination inaccordance with claim 1 wherein the electrochemically active materialsof said anode are zinc and cadmium and the electrochemically activematerial of said anode is a mixture of silver peroxide and silver oxide.